31 year old radiators are normally not helped by flushing. The small cooling tubes get plugged up and corroded to the point where they are beyond help. Also, your new engine probably exceeds the cooling capacity of your old cooling system. I would test drive it being careful to not overheat it. If it runs hot suggest putting in a larger capacity new radiator and modern electric cooling fans.......The engine compartment will really look great without the fan shroud and mechanical fan and the engine will run cool.